High purity germane gas has found wide use in the semiconductor industry. It is used in the fabrication of a wide variety of semiconductor devices, such as, transistors, diodes, integrated circuits, detectors, photovoltaic devices, and the like.
Despite this wide usage, the methods used to synthesize high purity, gaseous germane have hetertofore been characterized by the use of expensive reagents as starting materials or by low yields, or by both. None of the prior art syntheses obtain high yields from relatively inexpensive starting materials.
An early synthesis of germane (GeH.sub.4) was the reaction of magnesium germanide with dilute, e.g., hydrochloric acid added drop wise in a hydrogen atmosphere, or with ammonium bromide in liquid ammonia. Yields were low, less than 25%.
More recent synthesis routes utilize either germanium dioxide or germanium tetrachloride. The recent processes are of three types: (1) reduction of the germanium compound dissolved in alkaline media, or (2) reduction of germanium dioxide dissolved or dispersed in acidic media; and (3) reduction of germanium tetrachloride dissolved in tetrahydrofuran.
In the earliest of the modern practice prior art references, T. S. Piper and M. Kent Wilson, "The Preparation of Germane," J. Inorg. Nucl. Chem. 4, 22 (1957) prepared gaseous germane by dissolving germanium oxide in boiling hydrobromic acid. Their yield was 73% germane, of which 0.6% was digermane. T. N. Srivastava, J. E. Griffiths, and M. Onyszchuk, "Derivatives of Monogermane," Can. J. Chem. 40, 739 (1962), used the method of Piper and Wilson, but allowed the reduction to occur at 35.degree. C. instead of at 0.degree. C. They obtained yields of 90-95%. Griffiths, "Monogermanes-Their Synthesis and Properties," Inorg. Chem, 2, 375 (1963), used the same synthesis with a more complicated recovery method to obtain quantitative yields of up to 99.7%, with digermane constituting less than 1%. These references do not postulate an exact mechanism for the reduction reaction.
E. D. Macklen, "Preparation of Germane," J. Chem, Soc., 1989 (1959), added litium aluminum hydride to germanium tetrachloride dissolved in tetrahydrofuran at 35.degree. C. He obtained a 40% yield of monogermane under the best conditions. A side reaction is cited as the cause of the low yield. Formation of a yellow precipitate is mentioned. The proposed mechanism for the reaction is: EQU LiAlH.sub.4 +GeCl.sub.4 .fwdarw.GeH.sub.4 +LiCl+AlCl.sub.3
L. M. Antipin, "Reactions of Germanium Tetrachloride with Potassium and Sodium Tetrahydroborates," Russ. J. Inorg. Chem., 13, 162 (1968), used a solution of 4.6 grams of germanium tetrachloride dissolved in 30 ml water. To this solution was added drop wise a solution of 6.2 grams potassium tetrahydroboride dissolved in 120 ml water. A yield of 81.5% GeH.sub.4 was obtained when base was added at a mole ratio of 15:1. A yield of 96% was obtained using an "excess of borohydride," i.e., a ratio much greater than 15:1.
W. L. Jolly, "The Preparation of the Volatile Hydrides of Groups IV-A and V-A by Means of Aqueous Hydroborate," J. Amer. Chem. Soc., 83, 335 (1961), reported a 68% yield of germdane with around 2% digermane using 2 grams of 85% potassium hydroxide, 1 gram of germanium dioxide and 3.0 grams of potassium borohydride dissolved in that order in 25 cc of water. This solution was added to a 3.6M sullfuric acid solution. Jolly reported a brown germanium containing precipitate forming in the flask. At ratios of borohydride to GeO.sub.2 lower than 6:1, he reported enormous amounts of foam resulting from the formation of "soap like molecules" formed when too little reducing agent was present.
J. E. Drake and W. L. Jolly, "Hydrides of Germanium," J. Chem. Soc., 2708 (1962), used an aqueous alkaline solution of germanium dioxide in potassium borohydride. They reported a 73% yield under optimum conditions. Formation of the yellow polymer and vast amounts of foaming occurred when the acid used was hydrochloric acid or sulfuric acid. The 73% yield came from a reaction in acetic acid. Drake and Jolly found that the yield decreased when more than 3:1 borohydride was used.
John E. Drake, "The Preparation of Some Germanium Hydrides," University of California Lawrence Radiation Laboratory Berkeley, Calif., Contract No. -70405-ENG-48 (1961), used the method of Jolly, with an acid concentration of 3.0M, a base concentration of 7.4%, a GeO.sub.2 concentration of 0.40M, and a BH.sub.4 -concentration of 2.21M. In a series of experiments, he obtained yields of approximately 70%. He reported that the best results were obtained with the use of glacial acetic acid. He reported higher yields when polyglycol was added to the product to cut down on the foaming. Drake's postulated reaction for the reduction of the hydrated GeO.sub.2 is: EQU HGeO.sub.3.sup.- +BH.sub.4.sup.- +2H.sup.+ .fwdarw.GeH.sub.4 +H.sub.3 BO.sub.3
It has been heretofore impossible to obtain consistant yields of over 90% germane without the use of expensive reagents, such as hydrobromic acid or germanium tetrachloride, or unacceptably large amounts of the hydriding agent. Synthesis using relatively inexpensive reagents (germanium dioxide, potassium hydroxide, borohydride, and sulfuric acid used by Jolly, Drake and Jolly, and Drake) report relatively low yields, 70% or less. This inability to obtain high yields of germane by reactions using inexpensive reagents has resulted in high prices for high purity (claimed 99.9%) germane, e.g., of up to $25.00 per gram or more (August 1985 prices).